1. Field of the Invention
The present invention relates to an optical recording medium, which is capable of performing high-density recording and readout using short-wavelength laser beam. More particularly, it relates to a write-once type optical recording medium having excellent sensitivity and reliability according to DVD standard, which is capable of high-density recording and readout information by decreasing spot light using an organic dye thin film whose light reflectance changes with short-wavelength laser beam.
2. Description of the Related Art
Optical recording mediums capable of recording have come into wide use. The optical recording medium has a feature that the recording medium does not cause wear deterioration because a writing (recording) or reading (readout) head is of a non-contact type. Furthermore, the optical recording medium has attracted much attention and has been developed as a large-capacity information carrier because the writing density can be extremely increased by decreasing a spot size of optical beam.
The recording and readout of the optical recording medium are performed as follows. For example, the recording can be performed by condensing laser beam in a recording layer, converting said laser beam into heat energy and changing the properties of the recording film by melting, decomposition, removing, etc. to form a deformed layer pit, whereas the readout can be performed by a change in amount of reflected light as compared with the unrecorded portion. As the recording layer, a film of a chalcogenide metal represented by a tellurium alloy was put into practice at first. However, the chalcogenide metal film is disadvantageous in that it is harmful to humans, it formation involves a dry method and it is difficult therewith to meet increasing demand for higher density recording. Therefore, studies were made on recording layers comprising materials which could be used in place of the inorganic metal films including tellurium alloy films in order to obtain a recording medium having a dye film layer so that uniform pit diameters could be obtained for achieving higher density recording, higher sensitivity and dependence of laser beam on wavelength could be obtained, higher reliability could be obtained, and production costs could be reduced considerably. Therefore, proposals and reports about a medium using a recording layer containing an organic dye as a main component have been increased. The reason is as follows: (1) a recording layer can be made by forming a thin film using a wet method represented by a spin-coating method, although the reflectance is lower than that of the metal; (2) it is not corroded because of excellent oxidation resistance; (3) local heating can be performed without exerting a heat influence on the peripheral portion in comparison with the metal recording layer because of low heat conductivity; and (4) it has a lot of excellent properties; e.g. high sensitization can be realized.
From the structural point of view, a so-called air sandwiched structure obtained by providing an air layer on a recording layer of a generally used dye film and an optical recording medium capable of obtaining a readout signal corresponding to the CD standards are suggested. These structures are described in Japanese Patent Application Publication No. 3-759343, Japanese Patent Application Laid-open No. 2-87341, Japanese Patent Application Laid-open No. 5-67352 and Nikkei Electronics No. 469, page 107, Jan. 23, 1989.
In the optical recording disc in accordance with the CD standards, for example, there are known those discs obtained by forming a light absorption layer of an organic dye on a light transmitting resin substrate, forming a light reflection layer represented by Au directly or via a hard layer thereon and forming a resin protective layer on this light reflection layer. The light reflection layer is formed because high reflectance of 65% or more can not be obtained by using only the organic dye film.
In case that laser beam is irradiated to this disc, the organic dye layer absorbs light so that it is molten or decomposed and, at the same time, the substrate is softened and the dye is mixed with the substrate at the interface. Finally, a recorded pit whose interface is deformed is formed. In the deformed layer pit thus formed, the light reflectance varies with the light phase difference and reading can be performed similarly to CD.
As the organic dye layer of this optical disc, there are known, for example, squarilium dyes (Japanese Patent Application Laid-open Nos. 56-46221, 63-218398, 1-178494, 5-139047 and 7-44904), naphthoquinone dyes (Japanese Patent Application Laid-open Nos. 61-290092, 62-432, 63-168201 and 1-139047), azo dyes (Japanese Patent Application Laid-open Nos. 7-161069, 7-251567 and 8-99467), phthalocyanine dyes (Japanese Patent Application Laid-open Nos. 57-82094, 57-82095, 7-156550, 7-16068 and 7-52544) and cyanine dyes represented by the following general formula (III) (Japanese Patent Application Laid-open Nos. 59-24692, 6-199045, 7-262611, 6-338059, 6-320869, 2-87341, 62-201288 and Japanese Patent Application Publication 7-4981). ##STR2## wherein S.sub.1 and S.sub.2 independently represent an alkyl group, an aryl group or an alkoxyl group; W.sub.1 and W.sub.2 independently represent a halogen atom, a hydrogen atom, an alkyl group, an alkoxyl group, an aryl group, an alkoxysulfonyl group, a sulfonylalkyl group or a cyano group; and Z.sub.1 and Z.sub.2 independently represent a sulfur atom, an oxygen atom, a selenium atom or ethylene.
Among the above dyes, cyanine dyes represented by the general formula (III) above are mainly used in view of the sensitivity, C/N ratio, ease of converting into a thin film, etc. These cyanine dyes are generally dyes having absorption and reflectance at a semiconductor laser wavelength of about 780 to 830 nm, wherein the number of methylene chains in the center of the molecular structure is generally 2 (i.e., q is 2 in the general formula (III)) so as to be in accordance with the CD standards. However, the cyanine dyes having two methylene chains have a fatal problem that they cannot correspond to short-wavelength laser. Furthermore, it is considered that the cyanine dyes represented by the general formula (III) have problems about readout deterioration, long-term stability of the film, C/N ratio, jiter component, etc. Reasons for the above are (i) that heat accumulation arises in the recording layer by a long-term exposure to reading light and the dye itself is deteriorated and discolored, (ii) that the reading light is abosrbed at the interface of the recording layer where the deformed layer portion and non-recorded portion should be discriminated one from the other so that the melting or heat deformation arises slowly, and in addition distortion of the contour of the deformed layer pit arises simultaneously due to the similar heat accumulation, (iii) that when it is excited, the dye is oxidatively deteriorated (discolored) due to singlet oxygen which is generated due to transfer of energy from the dye to oxygen in the atmosphere, (iv) that as the phenomenon of deterioration after a long-term storage, there occur variation in the transmittance of a cyanine dye due to natural light and oxidation of the dye as well as association or agglomeration of the dye molecules due to oxygen or moisture, and the like. To solve these problems, various proposals have hitherto been made (cf., e.g., Japanese Patent Application Laid-open Nos. 62-201288, 62-201289, 57-66541, 59-124894, 59-203247, 62-133173, 63-198096, 59-21339, 57-11090, 60-44389, 60-71296, 63-1594, 57-11090, 5-38879, 7-262611, and 7-4981). However, these problems are still to be sufficiently solved.
On the other hand, there has been made development of an optical disc having high recording density in accordance with DVD standards as a high density optical memory. There have been proposed a system in which use is made of a semiconductor laser with a wavelength of about 600 to 680 nm, that is shorter than the wavelength used for currently prevailing Cds in view of the fact that the spot diameter is represented by the formula of [wavelength (.lambda.)/numerical aperture (NA) of an objective lens], a system in which the wavelength of light is shorten by utilizing a SHG (second harmonic generation) element and a beam spot diameter is decreased to a laser diffraction limit using an objective lens, thereby enhancing the recording density, and the like. Development of recorging media corresponding to these systems is desired.
Furthermore, there have been made an improvement in cyanine dye represented by the general formula (III) corresponding to the CD standards and proposal of a cyanine dye for shorter wavelengths are made (Japanese Patent Application Laid-open Nos. 6-199045, 7-186530, 8-306074, 5-38879, 6-40162, etc.).
However, various problems are still to be solved. For example, the recording layer does not have a sensitivity which corresponds to the desired wavelength. Further, the recording layer does not have a sufficient film stability since the number of methylene chain is one (q=1) in the cynine dye represented by the general formula (III) corresponding to the CD standards so that uniform stable amorphous film cannot be obtaned, or a reliability due to agglomeration or the like undesirable phenomena. Furthermore, the amount of the jiter component contained in a readout signal increases by heat interference between adjacent pits when higher density recording is attempted.